Every year, a huge crowd of basketwork eels gather at Patience Seamount, an underwater mountain off the coast of Tasmania. Now, for the first time, researchers have counted just how many of them show up.
Their results, published in Royal Society Open Science, suggest there were around 2.8 million eels populating the region in May 2023.
There were significantly lower numbers in March 2024 and April 2025, which the team chalks up to seasonal changes rather than a declining eel population. Numbers seem to rise as the year goes on, with the peak congregation size in May.
Read more: “Eels Don’t Have Sex Until the Last Year of Their Life”
The researchers didn’t count the eels one by one. Instead, they used underwater cameras in combination with sonar equipment that measured the echoes caused by reverberating sound waves.
While the sound waves helped identify how much activity was happening on Patience Seamount, visual observations and a small number of captured eels were essential in separating the eels from other types of fish (predominantly orange roughy).
That gave the researchers a figure of 3,900 U.S. tons (3,550 metric tons) of eel flesh biomass—or again, roughly 2.8 million eels.
The next question was: What are they all doing there? A ritualistic eel reunion? An eel conference? The researchers still aren’t completely sure, but the most likely explanation is that it’s a favored spot for spawning eggs.
Observations detailed in the study show the eels clustered together at a specific depth, rather than spreading out, suggesting the location and topography is important. “One possible driver is its proximity to the continental shelf, which may act as a physical boundary for bathyal migration or affect local circulation,” write the study authors. “Spawning in anguillid eels is influenced by currents and fronts, and seamounts play roles in eel reproductive ecology.”
The researchers also found that the female eels captured as part of the study had empty stomachs (suggesting they hadn’t been feeding), and eggs at a late stage of development, further supporting the spawning theory.
That said, the mystery isn’t completely solved yet. The study team didn’t directly observe the eels spawning or find eggs that had been released into the water. Nor is it clear why the fish prefer Patience Seamount, rather than, say, the 120 or so other underwater mountains in the same area.
Perhaps it’s nothing more than an old-fashioned case of just loving a spring vacation spot so much you can’t help but go back every year.
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Lead Image: Scoulding et al., Royal Society Open Science, 2026
